Sex in the cinema, when presented in full, fleshed-out detail, undoubtedly involves some degree of provocative intent. Popular filmmaking has not yet reached a point where scenes of explicit lovemaking don’t force viewers into an altered state, whether it be surprise, arousal or -- depending on the circumstances -- distaste. In fact, with the imposition of the Hayes Code now 40 years gone, the possibilities for using sex as a narrative device have only grown exponentially more complicated.

In Lust, Caution, Ang Lee’s cunningly effective new period piece, the two main characters contort and distend across bed sheets with serpentine intensity, their motions caught in unvarnished close-up. Every move in these hypnotically immersive scenes informs the story, imbuing it with an authentic sense of drama that gives the film a distinctly menacing tone. What might seem like gratuity is actually a strikingly eloquent form of psychological expressionism.

The sex serves a plot paralleling that of Army of Shadows, Jean-Pierre Melville’s bleak 1969 thriller about the French Resistance in the very relevant sense that both films track the efforts of dedicated WWII-era rebels and their incessant failures in the face of dictatorial opposition. Where Shadows centered on a tight knit group of French agents united against Nazi powers, Lust spotlights dedicated Chinese youth allied against the imposition of Japanese occupation. Specifically, Lee follows the quietly calculative activist Wang Chia Chi (Tang Wei), whose college affair with a politically driven colleague leads to her role in a dangerously fragile operation to seduce a powerful Japanese collaborator named Mr. Yee (the creepily withdrawn Tony Leung). Over the course of the film’s patient two hour, 37 minute running time, Wang marshals her aspiring actress chops for the slow process of seduction while her colleagues wait for the right moment to catch their prey. Like the sex scenes between Wang and Yee, during which the characters seem to continually build to climax before retreating, the resistance fighters are constantly frustrated by an inability to satisfy their strategic expectations.

The story grows increasingly unsettling -- and gripping -- as the fate of the clandestine operation remains impossible to predict. Despite Wang’s willingness to go all the way with her foe, she puts a valiant effort into remaining loyal to her group’s national pride, surpassing the pratfalls of emotional involvement that their intimacy inevitably brings to the surface. The enigmatic quality of their bedroom gyrations of the enemies is strengthened by Wang’s tenuous ability to mask what we assume to be a full-on performance.

But is it really a performance? In light of such ambiguity, another relevant text to consider in relation to Lust, Caution is Paul Verhoeven’s Black Book. In that film, a vengeful woman joins the Dutch Resistance and gets drafted to seduce an S.S. leader, unknowingly subjecting herself to a confounding scenario that finds her inadvertently in love with a Nazi. But Black Book relies on Verhoeven’s tendencies to turn subversive ideas into riotous cartoon entertainments. The same contradictory impulses inform the plot of Lust, Caution, but Lee gives them an atmosphere stuffed with an aesthetic of stark realism. Spy movies usually contain some degree of sensuality, but in this case the titillation is terrifying.